Chemistry › Definitions › Organic analysis

Organic analysis definitions

Every term the lessons in this unit define, lesson by lesson, worded as the lessons use them. Each lesson title links to its notes.

36 terms

Test-tube reactions for functional groups, and high-resolution mass spectrometry

Functional group: The atom or group of atoms in a molecule that is responsible for its characteristic chemical reactions.
Tollens' reagent: An aqueous solution of [Ag(NH₃)₂]⁺ that is reduced to silver by an aldehyde, giving a silver mirror, but does not react with a ketone.
Fehling's solution: An alkaline solution of a copper(II) complex that is reduced by an aldehyde to a brick-red precipitate of copper(I) oxide.
Molecular ion: The ion formed when a molecule loses one electron, M⁺; its m/z gives the relative molecular mass.
Precise relative molecular mass: The relative molecular mass calculated from the precise masses of the most abundant isotopes, given to four decimal places.
2,4-Dinitrophenylhydrazine: Brady's reagent, which gives a yellow or orange precipitate with an aldehyde or a ketone; the melting point of the purified precipitate identifies the carbonyl compound.
Triiodomethane test: Warming with iodine and aqueous sodium hydroxide; a pale yellow precipitate of CHI₃ shows a CH₃CO group or a CH₃CH(OH) group.
Fragmentation: The breaking of a molecular ion into a positive ion and a neutral radical, of which only the ion is detected.
Base peak: The tallest peak in a mass spectrum, given a relative abundance of 100.
M+1 peak: The small peak one m/z unit above the molecular ion, from molecules containing one carbon-13 atom; its height relative to M gives the number of carbon atoms.

Infrared spectroscopy: A technique that identifies the bonds in a compound from the wavenumbers of infrared radiation that they absorb.
Absorption: The removal from the beam of infrared radiation whose frequency matches the vibration frequency of a bond, seen as a dip in transmittance.
Wavenumber: The reciprocal of the wavelength, 1/λ, measured in cm⁻¹; it is proportional to the frequency of the radiation.
Transmittance: The percentage of the incident infrared radiation at a given wavenumber that passes through the sample.
Bond vibration: The stretching or bending of a covalent bond at a natural frequency set by the bond strength and the masses of the bonded atoms.
Fingerprint region: The region of an infrared spectrum below 1500 cm⁻¹, whose pattern of absorptions is unique to a compound.
Greenhouse gas: A gas whose molecules absorb infrared radiation emitted from the Earth's surface and re-emit it, warming the lower atmosphere.
Global warming: The rise in the mean temperature of the Earth's surface and lower atmosphere caused by increasing concentrations of greenhouse gases.

Chemical environment: A set of atoms in a molecule that are bonded to identical surroundings, so they absorb at the same chemical shift.
Chemical shift: The position of an NMR absorption relative to the TMS peak, δ, measured in parts per million (ppm).
TMS: Tetramethylsilane, Si(CH₃)₄, the reference compound whose single peak is defined as δ = 0.
Nuclear spin: The property of some nuclei, including every nucleus with an odd number of nucleons such as ¹H or ¹³C, that lets them align with or against an applied magnetic field.
Shielding: The reduction in the magnetic field felt by a nucleus caused by the electrons around it; a less shielded nucleus absorbs at a higher chemical shift.
Integration ratio: The ratio of the areas under the peaks in a proton NMR spectrum, equal to the ratio of the numbers of hydrogens in each environment.
Spin-spin splitting: The splitting of an NMR peak into lines by the hydrogens on adjacent carbon atoms, n neighbouring hydrogens giving n + 1 lines.
Deuterated solvent: A solvent such as CDCl₃ in which ¹H atoms are replaced by deuterium, so that it gives no peaks in a proton NMR spectrum.

Stationary phase: The phase that does not move, on or in which the components of a mixture are held, such as silica on a TLC plate.
Mobile phase: The liquid or gas that moves through or over the stationary phase and carries the components with it.
Adsorption: The holding of molecules on the surface of a solid by intermolecular forces.
Rf value: The distance moved by a component divided by the distance moved by the solvent front, both measured from the baseline.
Solvent front: The furthest point reached by the solvent on a chromatogram, marked as soon as the plate is removed from the tank.
Eluent: The solvent run continuously through a chromatography column as the mobile phase, carrying the components down it.
Retention time: The time taken for a component to pass from the injection point to the detector in gas chromatography.
Carrier gas: The unreactive gas, usually helium or nitrogen, that is the mobile phase in gas chromatography.
Calibration curve: A graph of peak area against concentration for standards of known concentration, from which the concentration in a sample is read.
HPLC: High-performance liquid chromatography, in which a liquid mobile phase is pumped at high pressure through a column packed with very small particles of stationary phase.